Fixed crash triggered by reading large files referenced in the clipboard.

- ServerItem::fromMimeData has functionality to parse .url. To that end
  files from the clipboard were read into memory to be parsed. However
  there was no restriction placed on file-size. If triggered while a big
  file was in the clipboard Mumble attempted to read all of it into memory
  and consequently crashed.
- Now limiting permissible file-size for such files to 10K which should be
  plenty and prevent future crashes.